The present invention relates to a control system for an internal combustion engine, and more specifically to a control system for controlling a fuel injection of an engine of a type having a stratified charge combustion mode and a homogeneous charge combustion mode.
Recently, the technique of in-cylinder direct fuel injection in a spark ignition engine such as gasoline engine is under development to improve the fuel efficiency and emission performance by using stratified charge combustion and homogeneous charge combustion.
In low and medium load regions, a control system for such a type operates an engine in a stratified combustion mode by injecting fuel directly into a combustion chamber during the compression stroke so as to produce a stratified combustible mixture only around the spark plug. The thus-achieved stratified combustion enables stable combustion with an ultra lean mixture, and hence significant improvement in the fuel efficiency and emission performance of the engine.
In a high load region over a predetermined engine load, the engine is operated in a homogeneous combustion mode to meet a demand for higher output torque. In the homogeneous combustion mode, fuel is injected during the intake stroke so as to produce a homogeneous air fuel mixture. (In some examples, a fuel injection valve for the homogeneous mode is provided separately in an intake port.)
The control system changes over the combustion mode between the homogeneous mode and the stratified mode in accordance with one or more engine operating conditions.
In an engine of such a type, a wide difference of the fuel injection timing between the homogeneous mode and the stratified mode causes difficulties. When, for example, a changeover command signal for commanding a changeover from the stratified mode to the homogeneous mode is generated between the homogeneous mode fuel injection timing and the stratified mode fuel injection timing for a certain cylinder of the engine, the control system tends to carry out the fuel injection twice in one cycle, first at the homogeneous mode fuel injection timing and then at the stratified mode injection timing. Such double fuel injection makes the air fuel mixture over-rich. In the case of changeover from the stratified mode to the homogeneous mode, the control system tends to miss the homogeneous mode injection timing and then inhibit the fuel injection at the stratified mode injection timing by responding to the changeover to the homogeneous mode. The failure of fuel injection makes over-lean the air fuel ratio averaged over the cylinders of the engine, and causes misfire or poor combustion.
An engine control system may be arranged to change a fuel injection mode in accordance with a selected combustion mode and a fuel injection timing. However, this control system cannot inject fuel adequately in some cases when the fuel injection timing is reached during a time interval between a judgement for changeover of the combustion mode and an end of calculation of a fuel injection quantity (fuel injection pulse width). In general, the judgement on changeover of the combustion mode and the calculation of fuel injection quantity are carried out in synchronism with time, whereas the counting to determine the fuel injection timing is synchronous with the crank angle. A priority is assigned to the crank-angle-synchronous operation. Therefore, the control system is unable to perform the fuel injection properly according to the request if the time has come for the injection during the process of calculation of the fuel injection quantity.